Light scattering experiments, particularly laser light scattering experiments, are commonly employed for measuring properties of certain physical systems such as solutions, microemulsions, micellar solutions, and colloidal dispersions. One common goal is to obtain data such as particle size, based upon diffusion coefficients. In measuring the scattering of laser light from such a physical system, it is important to measure not only the total (also referred to as "time-average"or "K-vector") intensity of the light scattered at a given fixed angle .theta., but it is also important to measure the intensity fluctuations of the light with time. The object of this invention and, indeed, other research, is to provide a single apparatus that can simultaneously attain both pieces of data.
The difficulty encountered in making both measurements simultaneously is that the gathering of total intensity data requires a very precise determination of the angle of scattering .theta., while the accurate collection of the intensity fluctuation data requires only a relatively accurate determination of .theta., but requires a very precise determination of the spatial resolution, that is, knowing exactly where in the sample plane the measurement is being made. This requirement for high spatial resolution arises from the fact that the observed intensity fluctuations observed arise from two different sources. The first source is called "homodyning" and is the interaction or interference caused by the scattering of light from separate particles in the physical system. This effect is the effect of interest to the researcher. The second source of fluctuations is called "heterodyning". It arises from light scattering at the wall of the sample holder, both at the wall-air interface and the wall-sample interface. Even a small error due to heterodyning can grossly affect the results in measuring fluctuation, although the net effect upon total intensity results is minimal.
The commonly accepted method of eliminating heterodyning effects is to immerse the sample cell in a liquid that has the same refractive index as the sample cell itself. This is referred to as use of an index-matching fluid. When the sample cell is constructed of glass, which is the most common case, the liquid of choice is toluene, which has recently come under intense scrutiny due to detrimental health effects. These effects are especially intolerable in a closed environment, as one would encounter in a space vehicle such as the space shuttle. Therefore, a method for obtaining the two distinct measures simultaneously using a single apparatus and not requiring a liquid such as toluene would be extremely beneficial for use in space applications.
A second, and not as common, method of eliminating heterodyning effects on the wall surfaces is to place an opaque mask in the middle portion of the optical train between two lenses used to focus and then refocus the image. Using such masks creates "dead zones" in the image which can be used to eliminate scattering from the walls of the sample cell. Although opaque masks are very effective when used in the center portion of the lens train, the best data point for collecting total intensity, that is, the line of light that passes directly through the middle of each lens on the light axis, becomes, unfortunately, unavailable, since it has been blocked by the opaque mask.
Another method of obtaining both pieces of data, that is, both total and fluctuation intensities, without use of an index matching fluid, is to use two separate optical trains. The obvious disadvantage to the researcher in this case is that the data are not simultaneously obtained, and inaccuracies due to changes in the light source, the sample, the angular position of the optical train, or other changes, are inevitably introduced.